rfc2913.txt

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Network Working Group                                           G. Klyne
Request for Comments: 2913                          Content Technologies
Category: Standards Track                                 September 2000


            MIME Content Types in Media Feature Expressions

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   In "A Syntax for Describing Media Feature Sets", an expression format
   is presented for describing media feature capabilities using simple
   media feature tags.

   This memo defines a media feature tag whose value is a Multipurpose
   Internet Mail Extensions (MIME) content type.  This allows the
   construction of feature expressions that take account of the MIME
   content type of the corresponding data.

Table of Contents

   1. Introduction .................................................. 2
      1.1 Terminology and document conventions ...................... 2
   2. Motivation and goals .......................................... 3
   3. MIME content type feature tag ................................. 3
   4. Examples ...................................................... 4
      4.1 Simple text ............................................... 4
      4.2 Fax image ................................................. 4
      4.3 Voice message ............................................. 4
      4.4 Web browser capabilities .................................. 5
   5. IANA Considerations ........................................... 5
   6. Security Considerations ....................................... 5
   7. Acknowledgements .............................................. 5
   8. References .................................................... 6
   9. Author's Address .............................................. 6
   Appendix A: 'Type' feature tag registration ...................... 7
   Full Copyright Statement ......................................... 9



Klyne                       Standards Track                     [Page 1]

RFC 2913       MIME Content in Media Feature Expressions  September 2000


1. Introduction

   In "A Syntax for Describing Media Feature Sets" [1], an expression
   format is presented for describing media feature capabilities as a
   combination of simple media feature tags, registered according to
   "Media Feature Tag Registration Procedure" [2].  This provides a
   format for message handling agents to describe the media feature
   content of messages that they can handle.

   This memo defines a media feature tag whose value is a MIME content
   type.  This allows the construction of feature expressions that take
   account of the MIME content type of the corresponding data.

   Note that a content type feature value may contain parameters, but
   this is discouraged.  See section 3 and appendix A, "Summary of the
   media features indicated" for discussion of this point.

1.1 Terminology and document conventions

   This section defines a number of terms and other document
   conventions, which are used with specific meaning in this memo.

   media feature
         information that indicates facilities assumed to be available
         for the message content to be properly rendered or otherwise
         presented.  Media features are not intended to include
         information that affects message transmission.

   feature set
         some set of media features described by a media feature
         assertion, as described in "A Syntax for Describing Media
         Feature Sets" [1].  (See that memo for a more formal definition
         of this term.)

   feature set expression
         a string that describes some feature set, formulated according
         to the rules in "A Syntax for Describing Media Feature Sets"
         [1] (and possibly extended by other specifications).

   This specification uses syntax notation and conventions described in
   RFC 2234, "Augmented BNF for Syntax Specifications: ABNF" [3].

      NOTE:  Comments like this provide additional nonessential
      information about the rationale behind this document.  Such
      information is not needed for building a conformant
      implementation, but may help those who wish to understand the
      design in greater depth.




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RFC 2913       MIME Content in Media Feature Expressions  September 2000


2. Motivation and goals

   The media feature expression syntax [1] and feature tags [2] were
   designed with a view to providing content media information that
   augments basic MIME content type information.  There are some
   situations where it is useful to be able include that content type
   information in a media feature expression:

   o  Media feature details may depend upon the content type being used.
      The media feature combining algebra and syntax [1] cannot apply to
      content type information unless it appears in the feature
      expression.

      For example, in HTTP 1.1 [4] with Transparent Content Negotiation
      (TCN) [5] acceptable content types and other media features are
      indicated in different request headers, with no clear way to
      indicate that they may be acceptable only in certain combinations.

   o  It is sometimes useful for all media capability information to be
      included in a single expression.  For example, DSN and MDN
      extensions [6] that allow a recipient to indicate media
      capabilities provide a single field for conveying this
      information.

   o  When media features are used to describe a message content, they
      may refer to inner parts of a MIME composite; e.g. the component
      parts of a 'multipart', files in a compressed archive, or
      encrypted message data.

3. MIME content type feature tag

   Feature tag name    Legal values
   ----------------    ------------
   type                <string>
                       containing a MIME content-type value.

   Reference: this document, appendix A.

   The 'type' feature tag indicates a MIME media content type (i.e.
   that appears in a 'Content-type:' header of the corresponding MIME-
   formatted data).  It must be a string of the form "type/subtype",
   where 'type' and 'subtype' are defined by the MIME specification [7].
   Only lower-case letters should be used.

   The content type must be given without any content-type parameter
   values.





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RFC 2913       MIME Content in Media Feature Expressions  September 2000


   To include information in media feature expressions that is otherwise
   conveyed in a MIME content-type parameter, a separate media feature
   tag should be registered [2] and used in the media feature
   expression.  This is illustrated by the use of 'charset' in the
   example at 4.1 below -- the 'charset' tag is defined by a separate
   registration [10].

      NOTE:  Allowing content-type parameters to be part of a type tag
      value was considered, but rejected because of concerns about
      canonicalization, ordering, case sensitivity, etc.  Only exact,
      case-sensitive, character matching is defined for media feature
      expressions [1].

4. Examples

4.1 Simple text

      (& (type="text/plain") (charset=US-ASCII)
         (color=binary) (paper-size=A4) )

4.2 Fax image

      (& (type="image/tiff")
         (color=binary)
         (image-file-structure=TIFF-S)
         (dpi=200)
         (dpi-xyratio=[200/100,200/200])
         (paper-size=A4)
         (image-coding=MH) (MRC-mode=0)
         (ua-media=stationery) )

4.3 Voice message

      (& (type="multipart/voice-message")
         (VPIM-version="3.0")
         (audio-codec=[G726-32,GSM-610])
         (audio-file-structure=[None,WAV])
         (ua-terminal=mobile-handset)
         (audio-channels=1) )

         NOTE:  in this case, some media features apply to MIME parts
         contained within the declared 'multipart/voice- message'
         content type.  The goal here is not so much to mirror the MIME
         structure as to convey useful information about the (possible)
         message content.






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RFC 2913       MIME Content in Media Feature Expressions  September 2000


4.4 Web browser capabilities

      (& (pix-x<=800) (pix-y<=600)
         (| (& (type="text/html") (charset=iso-8859-1)
               (color=limited) )
            (& (type="text/plain") (charset=US-ASCII) )
            (& (type="image/gif") (color=mapped))
            (& (type="image/jpeg") (color=full) ) ) )

   This example describes an HTML viewer that can deal with a limited
   number of color text tags, a gif viewer that supports mapped color,
   and a jpeg viewer that supports color.

5. IANA Considerations

   Appendix A of this document calls for registration of a feature tag
   in the "IETF tree", as defined in section 3.1.1 of "Media Feature Tag
   Registration Procedure" [2] (i.e. these feature tags are subject to
   the "IETF Consensus" policies described in RFC 2434 [9]).

   ASN.1 identifier 1.3.6.1.8.1.30 has been assigned by the IANA for
   this registered feature tag and has been placed in the body of the
   registration.